There are only two files needed for a sidebox: /includes/modules/sideboxes/whatever.php and /includes/templates/your_template/sideboxes/tpl_whatever.php.
So the only tricky part might be modifying the SQL code in the clone to do the custom select, and possibly creating some new constants in a language define file like /includes/languages/english/extra_definitions/your_template/whatever_defines.php.


If you want separate admin settings for number of alternate featured products or something, that would require some more code and database tweaking.